In the semiconductor light emitting device area, recently, a laser diode (LD) (hereinafter it calls a multi-wavelength laser) having a plurality of light emitting elements with different emission wavelengths on a common substrate has been actively developed.
The multi-wavelength laser is used for a light source of an optical disc apparatus, for instance. Generally, the semiconductor laser light with a wavelength of 780 nm band is used for reproducing CDs (Compact Disk) and for recording/reproducing writable optical discs such as CD-R (CD Recordable), CD-RW (CD Rewritable) and MD (Mini Disk) in the optical disc apparatus. In addition, the semiconductor laser light with a wavelength of 650 nm band is used for recording/reproducing DVDs (Digital Versatile Disks). Accordingly, mounting the above-described multi-wavelength laser on the optical disc apparatus enables to record/reproduce any existing plural kinds of optical discs.
As an example of such a multi-wavelength laser, a so-called monolithic two-wavelength laser diode in which two laser diodes with different emission wavelengths are mounted on one chip has been proposed. By using the monolithic two-wavelength laser diode for a common light source for reproducing/recording process in the DVD and the CD, it is possible to miniaturize an optical pickup and to simplify the adjustment of optical system.
A laser diode comprising an AlGaInP system red semiconductor laser device (hereinafter it refers to AlGaInP laser device) as a light source for the DVD and an AlGaAs system red semiconductor laser device (hereinafter it refers to AlGaAs laser device) as a light source for the CD is cited as the above-described two-wavelength laser diode.
FIGS. 7A and 7B show an example of a configuration of the two-wavelength laser diode. FIG. 7A shows a planar structure thereof and FIG. 7B shows a cross sectional structure thereof taken along the line II—II in FIG. 7A. FIGS. 7A and 7B are viewed from the main emitting plane side of the laser light. A two-wavelength laser diode 10 comprises a common substrate 11 made of GaAs, and an AlGaInP laser device 12 and an AlGaAs laser device 13 which are formed on the substrate 11. The substrate 11 is a so-called off substrate in which both sides thereof incline at a certain angle (off angle) with respect to the crystal axis direction. Here, both sides of the substrate 11 incline in a counterclockwise direction, that is, from a (0{overscore (1)}{overscore (1)}) plane 11a toward a (011) plane 11b with respect to the crystal plane (100) when viewed from a main emitting plane (0{overscore (1)}1) 11c. 
The off substrate suppresses the formation of natural superlattice to the laser devices 12 and 13 grown from the substrate and shortens the oscillation wavelength of the laser devices 12 and 13.
The AlGaInP laser device 12 is a red light emitting element having a oscillation wavelength of 650 nm and comprises an AlGaInP lower cladding layer, an active layer, an AlGaInP upper cladding layer, a GaInP layer, a GaAs contact layer and the like on the substrate 11 sequentially. The upper part of the AlGaInP laser device 12 is an air edge laser stripe 14 formed with current blocking regions 17 made of insulating layer on both sides thereof. On the other hand, the AlGaAs laser device 13 is a red light emitting element having a oscillation wavelength of 780 nm and comprises an AlGaAs lower cladding layer, an active layer, an AlGaAs upper cladding layer, a GaAs contact layer and the like on the substrate 11 sequentially. The upper part of the AlGaAs laser device 13 is a buried laser stripe 15 formed with an ion implantation layer made of B+ ion on both sides thereof.
The AlGaInP laser device 12 and the AlGaAs laser device 13 are placed so that the laser stripes 14 and 15 are parallel to each other. The AlGaInP laser device 12 is placed on the (0{overscore (1)}{overscore (1)}) plane 11a side from the centerline of the substrate and the AlGaAs laser device 13 is placed on the (011) plane 11b side from the centerline of the substrate, when viewed from the main emitting plane (0{overscore (1)}1) 11c of the laser light. The distance S of the centerlines of the laser stripes 14 and 15 is 120 μm.
As can be seen from FIG. 8, in the above-mentioned two-wavelength laser diode, an optical axis L1 of the AlGaInP laser device 12 and an optical axis L2 of the AlGaAs laser device 13 are deviated by the distance S viewed from the main emitting plane side.
Accordingly, when using the two-wavelength laser diode for a common light source for a reproducing/recording apparatus of DVD and CD, the switching a common optical system between the AlGaInP laser device and the AlGaAs laser device, that is, alignment of each optical axis with respect to the common optical system is required to read and rewrite in the DVD and the CD.
In order to align the optical axes of the AlGaInP laser device and the AlGaAs laser device with respect to the common optical system, the lens of the common optical system needs to be relatively shifted. The shift amount is preferably minimized in terms of the miniaturization of the entire apparatus. Therefore, in the conventional two-wavelength laser diode, the laser stripes 14 and 15 are placed to make the main emitting position of the laser light of the AlGaInP laser device 12 and the AlGaAs laser device 13 approach each other.
However, although two devices are placed so that the laser stripes 14 and 15 approach in the conventional two-wavelength laser diode, a certain isolation area is required and therefore, it has been difficult to further reduce the distance S between two devices.
Although the reasons will be described hereinafter, even two devices approach as close as possible, the optical axis L1 of the AlGaInP laser device 12 is deviated outside a central axis 14a of the laser stripe 14, so the distance between the optical axis L1 and the optical axis L2 becomes large, as shown in FIG. 8. This causes a problem that the shift amount of lens of the common optical system is larger than the distance S between the devices.
The present invention has been achieved in view of the above problems. It is a first object of the invention to provide a semiconductor light emitting device capable of reduction of the distance between the optical axes of two light emitting elements to minimize the shift amount of lens of the common optical system when used for the optical disc apparatus, and of miniaturization of the optical disc apparatus.
It is a second object of the invention to provide an optical disc apparatus capable of minimizing the shift amount of lens of the common optical system by using the semiconductor light emitting device of the present invention, thereby miniaturizing the optical disc apparatus.